Transcript environmnetal aspects of desalination

ENVIRONMENTAL ASPECTS OF
POWER-DESALINATION
I - AIR EMISSIONS
• Steam electric generating power/desalination plants
are designed to burn crude oil, gas-oil or heavy fuel
oil. The products of combustion released by fossil
fuel are ash particles, carbon dioxide (CO2), carbon
monoxide (CO), water vapor, Sulfur dioxide (SO2),
& nitrogen oxides (NOx).
II- ENVIRONMENTAL IMPACTS OF PLANT’S
INTAKE
There are two types of intake effects associated with powerdesalination operation.
1. Impingement effects: As the seawater going into the powerdesalination plant is screened & filtered, aquatic organisms are
removed from water. These organisms are either discharged or
returned back to the water body.
2. Entrainment effects: Smaller organisms passing through filters
find their way through the process they get exposed to chemicals,
higher temperature or pressure, conditions which are endangering
their existence.
Both of these effects may cause increased mortality rates for
planktons of all types as well as small fish. This in turn may
result in reduced population & reduced biodiversity.
III- ENVIRONMENTAL IMPACTS OF BRINE
WATER DISCHARGE
• Desalination plants separates saline water into two
streams: 1- A low dissolved solid concentration stream
(fresh water); and 2- brine reject (blow-down).
• The environmental impact of brine discharge depends to
a large extent on the physical, chemical & biological
characteristics of the receiving near-shore marine
environment.
• Large & modern desalination plants have sound designs
for their intakes & out-falls to ensure more efficient
operation & to avoid the passage of discharged effluents
to the feed water intakes.
The environmental impact of brine discharged to the near-shore
marine environment might be manifested in one or more of the
following forms:
1. Physical Impact: Resulting from the discharge of hot brine from
thermal desalination plants.
2. Chemical Impact: Resulting from chemical agents remaining in
the brine water & added for the control of biofouling, control of
calcium carbonate scale formation (sulfuric acid, polyphosphates
& polyelectrolytes) & antifoaming agents. RO necessitates
pretreatment of the feed water to avoid accelerated fouling &
scaling of the RO membranes.
3. Biological Impact: Biological impact is the secondary effect of
oxygen demand exerted by the natural & induced organics in the
brine water. The impact of BOD associated with lower levels of
DO in brine waters due to higher salt content & temperature will
ultimately reduce the level of DO in seawater adjacent to the
brine water out-falls.
IV- Environmental Impacts of Increased Salinity
on Open Seawaters
There has always been a myth among some environmentalists that
brine water discharged desalination plants inflicts damage to the
open near-shore marine environment. This is not the case for the
following reasons:
1. The amount of seawater withdrawn for desalination is relatively
minute when compared to the water mass of the open sea.
2. The amount & nature of salts discharged with the brine are
identical to the salt content of the open sea.
3. The concentration factor increases on the average by no more than
three. In case the brine is discharged after blending with cooling
waters, this ratio of three will significantly drop to near one.
4. In order to avoid re-circulation of plant effluents to the intakes,
the outlets are specifically engineered to discharge in coastal areas
where maximum hydrographic circulation disperse & dilute the
brine.
V- Impacts of Increased Salinity on Semi-
Enclosed Marine Environment
• The discharge of brine water in shallow & relatively
stagnant nearly-land-locked coastal areas such as bays,
khors, harbors, etc. result into a more pronounced
impacts on the encircled marine environment.
• The slight increase in salinity in the proximity of points
of brine water discharge is projected to be of limited
impact on the semi enclosed marine ecology of the Gulf.
VI- Environmental Impacts of Thermal
Pollution from Blow-down
• The temperature of the brine water effluent is typically above the
feed water temperature by 5 to 8 Co.
• In the life of marine organisms, temperature elevations from
ambient values causes thermal stress that result into an ecotoxicological effect.
• According to the US-EPA regulations, the allowable increase in
the weekly average temperature beyond 300 meters from the
point of discharge is 1 Co.
• The common practice is to ensure the minimization of these
effects, by selecting the plant sites & the engineering designs of
the discharge systems that expedite the dissipation of the thermal
inputs to the receiving marine environment.
VII- Environmental Impacts of Residual
Chlorine Oxidants in Brine Water
The bromide content of sea water ranges from 50 to 70 ppm, well
in excess of any chlorine dose applied in the desalination
operation. When chlorine mixes with seawater (pH=8.3), the
chlorine reacts in <10 sec. with the bromide ions to form
hypobromous acid
HOCL + Br - -------> HOBr + Cl• Fish were absent in water plumes having total residual chlorine >
0.05 ppm
• Photosynthesis of marine phytoplankton at 0.04 ppm of residual
chlorine was reduced.
• Discharge of trace levels of residual chlorine oxidants either in
open or enclosed seawaters will be very detrimental to aquatic life
in the nearshore marine environment.
IIX- Formation of Trihalomethanes (THMs) in
Brine Water
• The formation of THMs in brine water is a direct consequence of
free chlorine reaction with natural organics occurring in seawater
& other organic pollutants acting as precursors to form THMs.
Some of the volatile THMs species were found to be carcinogenic
& mutagenic to humans.
• The problem is further complicated by the fact that low boiling
point THMs can reach the desalination plant intakes & re-circulate
within the system. Once in the intakes, THMs will evaporate then
co-distill & concentrate in the potable water condensate. The
possible appearance of THMs in desalinated water can pose a
serious public health threat to consumers.
IX- Environmental Impacts of Trace Metals in
Discharged Brine Water
• In thermal desalination plants, it is plausible to find corrosion
products in brine waters resulting from the effect of water flow,
dissolved gases and treatment chemicals (acids) on the alloys
utilized in the construction of desalination pipes & equipment’s.
The corrosion products may include harmful heavy metals such as
Ni, Cu & Mo & less toxic metals such as Fe & Zn. Hg form
chlorine production was a serious problem.
• As conservative pollutants, metals will last in different
compartments of the marine environment forever. The level of
metals reflects the general status of the environment but it doesn’t
necessarily reflect the biological availability of these metals.
X- Environmental Impacts of Anti-scalants in Brine
Water
Alkaline scale forms in the desalination plants. The scales occur
when the bicarbonate ion breaks down by heat.
• The addition of H2SO4 breaks down the bicarbonate alkalinity &
prevent the calcium carbonate scales from forming. Environmental
impacts of acid additions is minimal due to the extremely large
carbonate buffering capacity of sea water.
• A scale inhibitor such as hexametaphosphate & surface active
agents like lignin sulphonic acid derivatives & esters of polyalkyl
glycols (Hagevap, Albrevap, Salvap) are added in desalination
plants to hamper the growth of carbonate & sulfate crystals.
X- Environmental Impacts of Anti-scalants in Brine
Water
For higher temperature scale inhibition, Belgard EV, Belgard
EV2000, Flocon 247 are used. These are polymers of organic acids
such as maleic anhydride & acrylic acids. The environmental
impact of polyphosphate in reject brine lies in its nutritional value.
When present phosphate causes a growth of plants. This excessive
plant growth usually means a reduction in diversity of species, and
result in an imbalance of food chain materials.
XI- Impact of (VLHs) on the Near-shore Marine
Environment
• VLHs are defined as compounds with boiling points
ranging between n-C6 & n-C14 normal & branched
alkanes, monocycloalkanes, aromatics and alkyl –
substituted analogues. It also include light aromatics
such as benzene & toluene. VLHs are considered to
be the most immediately toxic components of
petroleum other than the carcinogenic PNAHs.
• VLHs that can vaporize & consequently, co-distill
during the desalination process.